The pressure and temperature dependences of the impedance were measured, as a function of frequency (0.1Hz to 100kHz), in crystalline samples at 400 to 800C under pressures of 5, 10 or 20kbar (piston-cylinder apparatus). The material underwent a displacive first-order phase transition: from a trigonal structure (space group P31c) to a body-centered cubic structure having a high ionic conductivity (greater than 0.1S/cm at 10kHz). The bulk resistance was deduced from Argand plots at each temperature. A decrease in the electrical impedance indicated that the phase transition occurred at 5kbar (578C), at 10kbar (641C) and at 20kbar (764C). There was a small hysteresis in the phase transition temperature during cooling, but this hysteresis disappeared with increasing pressure. The slope which was estimated from electrical conductivity measurements of this phase transition was equal to 12.5C/kbar, and this corresponded to a Clausius-Clapeyron slope; with parameters of 26.4J/mol/K and 3.3cm3mol. In the extrinsic region, the conductivity had an activation energy which varied from 1.48eV (5kbar) to 1.35eV (20kbar). In the intrinsic region, the activation energy was 2.4 to 2.30eV. In the high-temperature conductive phase, the activation energy was 0.33 to 0.4eV and did not depend upon pressure. With increasing pressure, the temperature intervals of the extrinsic and intrinsic conductivities began to overlap. This resulted in a non-Arrhenius temperature dependence.

Ionic Conductivity and Pressure Dependence of the Trigonal to Cubic Phase Transition In Lithium Sodium Sulphate. N.Bagdassarov, H.C.Freiheit, A.Putnis: Solid State Ionics, 2001, 143[3-4], 285-96